PROJECT SUMMARY Different cognitive intervention approaches have been developed to attenuate decline (e.g., cognitive engage- ment, training, or stimulation), but it is not clear which approaches are efficacious. It is also not clear when, along the continuum of normal cognitive aging to mild cognitive impairment (MCI-a transitional stage before dementia onset), it is most effective to intervene. Crucially, the underlying mechanisms of effective cognitive in- terventions are not known; a critical barrier to progress in this field. There is an urgent need to identify effective approaches to curb the increasing prevalence of dementia. The overall objective is to apply the best cognitive interventions to attenuate cognitive and functional decline and curb dementia prevalence. The long-term goals are to promote maintained health and independence among older adults. [This randomized clinical trial will de- termine the efficacy of a novel cognitive engagement intervention approach (intense piano training) as com- pared to cognitive stimulation (which will serve as a stringent, active control)]. Grounded in theory, the central hypothesis is that interventions enhancing central auditory processing (CAP), a strong, longitudinal predictor of MCI and dementia, will improve cognition. Music training is increasingly recognized as a feasible means to at- tenuate age-related cognitive decline. Prior research and our preliminary data suggest that intense piano train- ing may enhance CAP and is likely more effective than cognitive stimulation. [Correlational studies indicate su- perior CAP, executive function, and other cognitive abilities for adults with formal music training compared to non-musicians. Our results across three pilot randomized trials show that piano training improves older adults' CAP and executive function, including those with MCI]. The specific aims of the study are to examine the effi- cacy of intense piano training relative to cognitive stimulation (active controls) to improve CAP, cognition, and everyday function among older adults with and without MCI. The efficacy of piano training will be established and moderating effects of MCI status will be examined. The proposed study further aims to elucidate the un- derlying mechanisms of effective cognitive intervention approaches by exploring mediators of training gains. Study outcomes will be identification of effective intervention approaches to attenuate age-related cognitive and functional decline and elucidation of the mechanisms. The proposed study is innovative, in our opinion, as the first phase II randomized trial of piano training to enhance older adults' cognition. Moreover, this study is innovative by including older individuals with and without MCI and assessing effects on everyday functional performance. Most importantly, to advance this field, we will use mediation analyses to elucidate the underlying mechanisms of intervention effects. The contributions will be significant, advancing our understanding of how cognitive interventions produce cognitive and functional change. Results will facilitate improved design and im- plementation of effective interventions to attenuate cognitive decline and curb the prevalence of dementia, thereby improving public health.